The development of lighting devices has focused in many ways on how to extract as much light as possible into the ambient and while doing so provide at least some directionality of propagation to the light to make it useful for application in space illumination, indication, display and/or other lighting applications. Such aspects apply to all types of SSL and non-SSL lighting devices and generally manifest themselves in the design of the optical subsystem. These requirements are particularly relevant when light is generated within optically dense material. Efficient utilization of high brightness light that originates from quasi-point sources and controlling respective glare provides a number of challenges for optical subsystem design. These and other aspects have become increasingly important in the configuration of light-emitting diodes (LEDs), LED-based lighting devices and other SSL devices.
SSL devices in particular are finding rapid adoption in large portions of illumination applications due to their low power consumption, high luminous efficacy and longevity in comparison to incandescent and fluorescent light sources. SSL devices have been developed that can generate quality white light via down-conversion of short wavelength pump light, including ultraviolet, blue or other light provided by corresponding LEDs, via a suitable luminescent material (also referred to as a phosphor). Such devices may be referred to as phosphor-based LEDs (PLEDs). Although subject to losses in efficacy due to light-conversion, various aspects of PLEDs promise reduced complexity, better cost efficiency and durability of PLED-based luminaires in comparison to luminaires that generate white light from light emitted by various combinations of LEDs that directly generate red, green, blue, amber and/or other colors of light, for example.
While new types of phosphors are being actively investigated and developed, configuration of PLED-based lighting devices and/or luminaires, however, provides further challenges due to the properties of available luminescent materials. Challenges include light-energy losses from photon conversion, generally referred to as Stokes loss or Stokes shift, self-heating from Stokes loss, dependence of photon conversion properties on operating temperature, degradation due to permanent changes of the chemical and physical composition of phosphors in effect of overheating or other damage, dependence of the conversion properties on intensity of light, propagation of light in undesired directions in effect of the random emission of converted light that is emitted from the phosphor, undesired chemical properties of phosphors, and controlled deposition of phosphors in lighting devices, for example.
Therefore there is a need for a lighting device that overcomes at least one of the deficiencies of the state-of-the art.